Rheology modifiers, which are commonly referred to as sag control agents, can be an important component of a curable composition, particularly a coating composition. By controlling flow and sag of the coating, the rheology modifiers allow for the deposition of coating with sufficient thickness to impart the necessary durability while maintaining the desired appearance, i.e., gloss, distinctness of image, and smoothness.
Sag control agents such as those described in U.S. Pat. Nos. 4,311,622, 4,677,028 and 4,851,294 are the reaction products of an amine and a diisocyanate, optionally reacted in the presence of a resinous binder. These sag control agents are typically crystalline in nature and their effectiveness is dependent on size and shape of the crystals as well as their concentration. Although the above-identified references disclose the use of these rheology modifiers in coating compositions to provide improved sag resistance, the polymers associated with these coating compositions are mostly hydroxyl functional.
It has been found that the inclusion in compositions, particularly curable coating compositions, of a rheology modifier which is the reaction product of an amine and an isocyanate in conjunction with a polymer having carbamate, urethane and/or amide functional groups provides a greater degree of thixotropy and sag resistance superior to that provided by the inclusion of the rheology modifier with a polymer having hydroxyl functional groups.